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    <script>
        var a = 10;
        ++a; //++a 结果为11 a=11
        var b = ++a + 2; //++a 结果为12
        console.log(b); //14

        var c = 10;
        c++; //c++ 结果为11 c=11
        var d = c++ + 2; //此时c++先返回原值即c++ = 11 执行完这行代码c才等于12
        console.log(d); //13

        var e = 10;
        var f = e++ + ++e; //1.e++ = 10 e = 11 2.e=11 ++e = 12
        console.log(f); //22
        //后置自增 先表达式返回原值 后面变量在自加1
    </script>
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